Frontotemporal dementia (FTD) is the second most common early-onset neurodegenerative disease with cognitive impairment, and almost one half of FTD cases are thought to be familial. FTD linked to chromosome 3 (FTD3) is autosomal dominant and strongly linked to mutations in the CHMP2B gene, which encodes a subunit of the endosomal sorting complex required for transport-III (ESCRT-III). At late endosomes, ESCRT-III catalyzes membrane scission reactions through its assembly process, and CHMP2B sustains this activity by stimulating the ATPase that catalyzes disassembly of ESCRT-III, thereby replenishing the availability of subunits for repeated rounds of complex assembly. In FTD3 patients, point-mutation of one genomic CHMP2B locus results in truncation of its coding sequence, which eliminates the ATPase-binding site and simultaneously removes an autoinhibitory region. Truncated CHMP2B expression results in genetic-dominant inhibition of ESCRT-III membrane scission activity, but the mechanistic basis for this dysfunction is unknown, nor is it understood how it results in the accumulation of late endosomes and autophagosomes, which is the predominant cellular pathology in FTD3. The long-term objective of this research is to understanding the mechanistic basis for late endosomal dysfunction in FTD3 using the budding yeast Saccharomyces cerevisiae as a model system. The central hypothesis is that CHMP2B truncation inhibits the fusion of late endosomes and autophagosomes with lysosomes, which normally prevents these organelles from accumulating. This research will study heterozygous diploid yeast in which one copy of the CHMP2B gene ortholog is truncated in a manner homologous to that described in a large Danish kindred afflicted with FTD3. The methodology to be used includes protein biochemistry, localization studies, expression profiling, and functional assays to study how the FTD3 truncation affects ESCRT-III and the machinery that regulates lysosomal fusion activity. The rationale for this research is that these parameters must be determined in order to develop a strategy for defining the critical protein interactions and specific regulatory elements that link endolysosomal fusion to ESCRT-III function. Using yeast as a model system to understand this relationship is directly relevant to human health because the mechanisms of endolysosomal fusion and ESCRT-III function are highly conserved. With respect to expected outcomes, it is anticipated that completion of this research will reveal the nature of ESCRT-III dysfunction in FTD3 and how it results in accumulation of late endosomes and autophagosomes. Such results are expected to have an important positive impact because they will yield fundamental insight into a poorly understood regulatory step in the endocytic pathway, reveal potential targets for the prevention and treatment of FTD3, and inspire new and innovative approaches to understand the mechanisms of endosomal dysfunction that characterize many other neurodegenerative diseases.